Intermediate-frequency amplifier



A ril 26, 1938. 'r. J. WEYERS- INTERMEDIATE FREQUENCY AMPLIFIER Filed June 15, 1955 INVENTOR THEODORUS WEYERS BY 7 ATTORNEY Patented Apr. 26, 1 938 INTERMEDIATE-FREQUENCY AMPLIFIER Theodorus Josephus Weyer s, Eindhoven, Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands In Germany June 18, 1934 Application June 15, 1935, Serial No. 26,772

4 Claims. (01. mpn

The invention relates to a circuit arrangement for intermediate-frequency amplifiers such as they are employed in superheterodyne receiving apparatus.

In such amplifiers the successive amplification stages are usually coupled to one another by means of band filters which have such frequency characteristic curves that, dependent upon the side-band frequencies of the intermediate-frequency carrier wave, a determined band of frequencies is almost uniformly transmitted and that all the frequencies located above and below said band are suppressed. As, however, the damping of the usual band filters does not increase rapidly enough at the limits of the transmitted band the selectivity of the intermediatefrequency amplifier is often insufficient which results in disturbances in the reception of a weak transmitter due to neighboring strong transmitters. In order to remove these drawbacks it has previously been proposed to render the width of the band of the band-filter adjustable in such manner that in the reception of a weak transmitter the Width of the band of frequencies which are almost uniformly transmitted, is reduced.

According to the invention an improved intermediate-frequency amplifier with which the width of the band of frequencies is transmitted with almost uniform amplification may be obtained by coupling two or more amplification stages by means of transformers tuned on both the primary and the secondary side. In this case the coupling between the windings may be adjusted in such manner that with the tightest adjustment of the coupling the windings of one transformer are over-critically coupled and are slightly damped and the windings of another transformer are critically coupled and are strongly damped, whereas with the loosest adjustment of the coupling the first-mentioned transformer windings are critically and the last-mentioned transformer windings are less than critically coupled.

The invention will be explained more fully with reference to the accompanying drawing in which Figure 1 represents, by way of example, a circuit arrangement according to the invention, and Figures 2 and 3 represent resonance curves to help explain the circuit of Figure 1.

In Fig. l of the drawing, the anode l of an intermediate frequency amplifying tube or of the first detector is connected to the primary winding 3 of an intermediate-frequency transformer I whose secondary winding 4 is located in'the control grid circuit of an intermediate-frequency amplifying tube 6. Both windings are tuned by means of condensers 2 and 5. The tube 6 is connected in a similar manner by means of a trans former II having tuned primary and secondary to a following intermediate-frequency amplifying tube or to a rectifier or second detector 7.

The coupling between the'windings of both transformers is adjustable in unison as indicated by the arrows and the dotted line joining them and, in accordance with the invention, it is so chosen that with the tightest adjustment of the coupling the windings of transformer I are overcritically coupled and are slightly damped whereas the windings of transformer II are critically coupled and are strongly damped. By critical coupling it is to be understood to mean any coupling with which the-resonance curve just does not yet show a sag. Under these conditions the course of the resonance curves of the tuned transformers I and II may be represented by the curves l and 2 respectively in Figure 2, so that the resonance curve of the amplifier takes the shape of curve 3. Furthermore, according to the invention, with the loosest adjustment of the coupling of the windings of transformers I and II the former transformer windings are critically coupled and the latter are less than critically coupled. The resonance curves have in this case the shape shown in Figure 3.

When the coupling is altered from the state in which it is tightest into the state in which it is loosest, the resonance curve 3 of Figure 2 gradually changes into the resonance curve 3 of Figure 1 3, which is attended with an increase of the selectivity of the amplifier. v

- In the case of disturbances in the reception of a weak transmitter due to interference by neighboring transmitters, these disturbances can be suppressed by loosening the coupling between the windings of the transformers-I and II.

As the windings of transformer I are only slightly damped and are over-critically coupled, the ratio between the voltage across the secondary winding and the voltage across the primary Winding increases the more, the looser the coupling is made. Onthe other hand, the windings of transformer II-are strongly damped and are less than critically coupled so that with a loosening of the coupling the ratio of the voltage across the sec-, ondary winding to the voltage across the primary winding decreases. The adequate choice of the dampings and couplings allows of obtaining in this case a mutual neutralization of the variations of the voltage transmission of both transformers so that when the coupling varies the 40 .quencies j and fzcanbe derived.

' total amplification' of the intermediatefrequency Y-amplifierremains substantially :constant; "In' other words, the coupling of the transformer I changes "between o-ver -critical andcritical coupling, so that this coupling is 'ove'r-'-criticalover j the Wholejcontrol range except for'the, loosest coupling obtainable. Thefcoupling of thetran's former II changes between critical. and undercritical coupling, 'so I that "thistransformer is underrc'riticaHy coupled over the 'whole control range except for the tightest coupling obtainable. Therefore the voltage ratio'between theseco nrh" ary and the primary windings increases for transformer I; and decreases for transformer Has the can be derivedfrom the resonance curve I in V r and therefrom the damping of I the tuned circoupling is loosened, so that a constant amplificationover the whole control range may be';0b.-

tained. It isto be understood that'the' expression damping of the tuned circuits as it is used herein'is defined as theratio of V HWL 1 H r being the totalresistance; w, the resonant frej :quency; and L the inductance of the tuned circuit under consideration. By total resistance .1

,is not only meant the resistance of the tuned circuit' itself but also the parallelresis-tance formed by the internal resistance 'ofthe tube to "which the circuitis connected. As to the tuned circuits forming the over-critically coupled band filter indicated by-IinjFig. 1, the ratio j a j isimade as-smallas-can be realized in practice with the usual; coils. condensers and amplifier tubes, so that a humped-resonance curve is obtained as shown at. l in Fig.2. From this resonance curve the amplitude hr at the mid-frequency'fm and the amplitude hz at theupeak fre 'I'he tuned circuits forming the bandfilter'II are critically .coupledso that tn r sonaneecurve. ofthis band filterl will. havelthe form-shown by the curve'l2' in Fig. 2. '(Itis to be understood that .the several resonancelcur ves.illustrate merely vcuits can be determined. ,In'a practical example obtained with the following The damping necessary in the sec ond'pair; of I tuned circuits can easily be obtainedby proper choice of the wire of which the coils are wound or by means o-f additional resistances in seriesor parallel to the circuits.

What I claim is:

V 1. An intermediate-frequency amplifier com prising aplurality of tubes, a firstipairof @adjustably coupled tuned. circuits connected betwee'n successivetubes, a second pairof adjustably-coup led tuned circuits connected between F successive tubes; characterized in that with the tightest. adjustment of the coupling, one pair'of tunedgcircuits is over-critically coupled and is slightly damped and the'other pair of tuned circuits is critically coupled and is strongly damped,

whereas-,.wi th the loosest adjustment ofthe coupling; the firs't mention'ed pair of tuned circuits is critically coupled and the'last-mentioned pair is less than critically coupledgand further-Chara 'acterized in that means are providedfbr effect ing the coupling adjustments of the twopairs or coupled'circuits in unison whereby constant am-- plification I over the Whole control range is obtained. 1 e

of vacuum tube amplifiers and means ior vja'rye 'ing'the coupling between the'respective pairs of; tuned circuits in unisonsuchthat for one adjustrrient one of said pair of tunedcircuits is over- 7 critically coupled and the other is critically cou(-, pled and for another adjustment the former pair of circuits is critically coupled and thelllatter pair is less than critically coupled.

" 3 ystem for the transmission ofj a band of frequencies with substantially uniformamplificw tioncdmprisinga first pair of tuned coupled cir 2. 'A circuitarrangement'comprising apair of.

. fixedly tuned but variably coupled circuits inter- I connected between a pair of vacuum tube am- V plifiers, a second pair of fixedly tuned but variably coupled circuits interconnected between a pair-30.

cuits, means foryarying the coupling between i said circuits through a rangefromover-critical coupling jto critical coupling, asecond pair. of

similarly tuned coupled circuits, means for vary:

ing the "coupling between saidsecond pair ofcircuits through arange from. critical coupling to 'less than critical coupling, and means tosimultaneously vary said coupling means through their respective ranges.

I 4. The method of controlling the selectivity of an amplifier provided with two pairs of similarlytuned coupled circuits which consists in overcritically coupling, one pair of said circuits and n simultaneously critically coupling the other pair whereby the net result of the amplifier is to transmit a'comparatively wide band of frequencies,

and then critically coupling the first pair of circuits and simultaneously coupling the second pair to less than critical whereby the net result of the amplifier is to transmit a comparatively" narrow band of frequencies.

'IV'HEODORUS/JOSEPI-IUS WEYERSL V 

